1. Field of the Invention
The present invention relates to the technical field of a power conversion circuit for light emitting diode (LED) illumination equipments, and more particularly to an LED driver circuit for supplying a triode for alternating current (TRIAC) holding current by using a controllable current source to assure that a TRIAC component obtains a current greater than the current required for the operation of the TRIAC component when the TRIAC component is conducted regardless of the light emitting brightness of an LED light source, so as to enhance the operating stability and dimming accuracy.
2. Description of the Related Art
In general, most of the control circuits for LED lamps employ a switch such as a silicon controlled rectifier (SCR) TRIAC to control different phases in order to change the voltage phase of an input current, and a driving voltage outputted by the control circuit is changed by the method of switching the conduction angle of the voltage phase to adjust the driving current passing into the LED light sources and achieve the dimming effect. Although the dimming devices of this sort have the advantages of simple control and easy installation, the voltage waveform of the input power is situated at a distortion status to cause the problems of a low PF and an increased harmonic voltage. From the current/voltage (I/V) characteristic curve of the LED, the LED is a nonlinear component; in other words, the voltage and the current are not directly proportional to each other. Therefore, the dimming effect of the aforementioned dimming method is inaccurate which results in unnecessary power consumption due to the inconsistent driving voltage and driving current. Under the influence of the characteristics the TRIAC element, if the operating voltage passing into the control circuit is too low and the current passing through the TRIAC current element is insufficient, the TRIAC will repeat switching the operating status, and the driving current will not be continuous to blink the LED and further lower the illumination quality.
With reference to FIG. 1 for the circuit diagram and waveform diagram of a conventional LED driver circuit with a holding current, the driver circuit 1 comprises a dimmer 10 which is a TRIAC element, a bridge rectifier 11, a holding switch 12, a holding resistor (RH) 13 and a filter capacitor 14, and the bridge rectifier 11 is coupled to an external power supply through the dimmer 10 to rectify and form an input voltage (Vin) and an input current (Iin). After the filter capacitor 14 receives the input current and converts and forms a driving current (IL), the driving current is supplied to the LED. In addition, the holding switch 12 is a N-Type Metal Oxide Semiconductor Field-Effect Transistor (N-MOSFET) having a drain coupled to the bridge rectifier 11 for receiving an input current, a source coupled to the holding resistor 13, and a gate having a holding voltage (VH), so that the holding resistor 13 receives the holding voltage through the holding switch 12 to output a holding current (Ihold), wherein Ihold=(VH−VGS_N−MOSFET)/RH, and the dimmer 10 is maintained at a stable operating status to improve the dimming accuracy. However, the holding switch 12 and the holding resistor 13 undoubtedly cause a power loss and affect the overall working efficiency of the circuit, and thus resulting in a low power utilization rate and failing to comply with the user requirements of the present electric appliances.
In view of the aforementioned problems of the prior art, the present invention intends to improve the circuit architecture of the conventional driver circuits, so that the driving current is maintained at a constant current while reducing power consumption effectively. To cope with the physical properties of the LED, the simple circuit architecture is employed to achieve the effects of compensating the input voltage, improving the overall operating quality of the circuit, and enhancing the light emitting power of the LED.